Analysis of heterointerface recombination by Zn1−xMgxO for window layer of Cu(In,Ga)Se2 solar cells

An adjustment of a conduction band offset (CBO) of a window/absorber heterointerface is important for high efficiency Cu(In,Ga)Se₂ (CIGS) solar cells. In this study, the heterointerface recombination was characterized by the reduction of the thickness of a CdS layer and the adjustment of a CBO value by a Zn_1−xMg_xO (ZMO) layer. In ZnO/CdS/CIGS solar cells, open-circuit voltage (V_oc) and shunt resistance (R_sh) decreased with reducing the CdS thickness. In constant, significant reductions of V_oc and R_sh were not observed in ZMO/CdS/CIGS solar cells. With decreasing the CdS thickness, the CBO of (ZnO or ZMO)/CIGS become dominant for recombination. Also, the dominant mechanisms of recombination of the CIGS solar cells are discussed by the estimation of an activation energy obtained from temperature-dependent current-voltage measurements.     

Reduction in surface recombination through hydrogen and 1-heptene passivated silicon nanocrystals film on silicon solar cells

We demonstrate reduction in surface recombination by integrating silicon (Si) nanocrystal layer on single crystalline Si solar cell. Si nanocrystals (NCs) are grown by electrochemical etching of (1 0 0) oriented p-type Si wafer. The substructures on the substrate are extracted and passivated it with hydrogen and 1-heptene molecules. Colloidal dispersion of Si NCs was spin casted on solar cell at room temperature. Apart from the I–V curve depicting the efficiency of solar cell, diffuse reflectance, measurement of short circuit current as a function of wavelength and current–voltage characteristics of solar cell were recorded with and without NCs layer. The analysis showed 9.4% increase in Si solar cell efficiency due to the surface passivation effect offered by Si NCs. Measurements of surface recombination time confirms the improved passivation by NCs.     

Comparison of various sol-gel derived metal oxide layers for inverted organic solar cells

Inverted bulk-heterojunction solar cells have recently captured high interest due to their environmental stability as well as compatibility to mass production. This has been enabled by the development of solution processable n-type semiconductors, mainly TiO₂ and ZnO. However, the device performance is strongly correlated to the electronic properties of the interfacial materials, and here specifically to their work function, surface states as well as conductivity and mobility. It is noteworthy to say that these properties are massively determined by the crystallinity and stoichiometry of the metal oxides. In this study, we investigated aluminum-doped zinc oxide (AZO) as charge selective extraction layer for inverted BHJ solar cells. Thin AZO films were characterized with respect to their structural, optical and electrical properties. The performance of organic solar cells with an AZO electron extraction layer (EEL) is compared to the performance of intrinsic ZnO or TiO_x EELs. We determined the transmittance, absorbance, conductivity and optical band gap of all these different metal oxides. Furthermore, we also built the correlations between doping level of AZO and device performance, and between annealing temperature of AZO and device performance.     

Impact of Zn1-xMgxO:Al transparent electrode for buffer-less Cu(In,Ga)Se2 solar cells

Buffer layers such as CdS and ZnS are used in high efficiency Cu(In,Ga)Se₂ (CIGS) thin film solar cells. Eliminating buffer layer is attractive to realize low-cost thin film solar cells by reducing fabrication process. However, the elimination of the buffer layers leads to shunting due to the interface recombination between transparent conductive oxide (TCO) and CIGS layers. To reduce the interface recombination, the control of conduction band offset (CBO) is effective. In this study, we fabricated Zn_1−xMg_xO:Al (ZMO:Al) as the TCO for the CBO control. ZMO:Al was prepared by co-sputtering of ZnO:Al₂O₃ (ZnO:Al) and MgO:Al₂O₃ targets. ZMO:Al shows high transmittance in visible region and the band gap energy widen with the addition of Mg to ZnO:Al. Buffer-less CIGS solar cells with an Al/NiCr/TCO/CIGS/Mo/soda-lime glass structure using ZMO:Al and ZnO:Al were fabricated. For comparison, ZnO/CdS buffered cell was also fabricated. Current density-voltage characteristics of the devices showed the cell with ZMO:Al film achieved higher efficiency compared to the buffer-less cell with ZnO:Al. This result suggested that the control of CBO is important to reduce interface recombination between TCO layer and CIGS absorber.     

Stable and highly efficient MoS2/Si NWs hybrid heterostructure for photoelectrocatalytic hydrogen evolution reaction

We report, the fabrication of molybdenum disulphide (MoS₂) wrapped silicon nanowires (Si NWs) for visible light driven water splitting applications. The morphological and elemental studies ensure the vertical alignment of Si NWs wrapped with 2D layered MoS₂. The photoelectrocatalytic (PEC) results evidence the significant enhancement in performance of MoS₂/Si NWs based hybrid photocathode with ~300 mV (under reversible hydrogen electrode (RHE)) anodic shift in onset potential as that of pristine Si NWs (+0.194 V vs. RHE), and the current density of −26.5 mA/cm² was achieved at the applied bias of 0 V vs. RHE. Further, the electrochemical impedance studies ensure the interface resistance-free charge transfer between Si NWs and electrolyte via 2D MoS₂ layer which provokes rapid hydrogen production. The wrapping of Si NWs with MoS₂ protects the superlative photocathode from harsh acid electrolyte environment. The overgrown MoS₂ triangular particles with active sulphur edge sites are found to eventually augment the solar hydrogen evolution rate. Further, the PEC performance of our MoS₂/Si NWs is also comparable with stable Pt/Si NWs photoelectrode. It is note-worthy that, MoS₂/Si NWs hybrid heterostructure would be a potential candidate in future large scale, low cost and day-to-day solar water splitting applications.     

Thickness effect of RF sputtered TiO2 passivating layer on the performance of dye-sensitized solar cells

We report on the characteristics of a TiO₂ passivating layer grown by radio frequency (RF) magnetron sputtering on F-doped SnO₂ (FTO) electrodes as a function of its thickness. The optical transparency, surface roughness and passivation properties of the TiO₂ layer passivating the FTO electrode depend on the thickness of the TiO₂ passivating layer. In addition, it was found that the power conversion efficiency of the dye-sensitized solar cells (DSSCs) is critically dependent on the thickness of RF sputtered TiO₂ layer inserted between FTO electrode and nanoporous TiO₂ layer. The DSSC fabricated on 50 nm thick TiO₂ passivating FTO electrode showed the maximum power conversion efficiency of 4.42% due to effective prevention of the electron transfer to electrolyte. This indicates that the thickness optimization of the TiO₂ passivating layer is one of the important parameter to obtain high performance DSSCs.     

Influence of using amorphous silicon stack as front heterojunction structure on performance of interdigitated back contact-heterojunction solar cell (IBC-HJ)

Interdigitated back contact-heterojunction (IBC-HJ) solar cells can have a conversion efficiency of over 25%. However, the front surface passivation and structure have a great influence on the properties of the IBC-HJ solar cell. In this paper, detailed numerical simulations have been performed to investigate the potential of front surface field (FSF) offered by stack of n-type doped and intrinsic amorphous silicon (a-Si) layers on the front surface of IBC-HJ solar cells. Simulations results clearly indicate that the electric field of FSF should be strong enough to repel minority carries and cumulate major carriers near the front surface. However, the over-strong electric field tends to drive electrons into a-Si layer, leading to severe recombination loss. The n-type doped amorphous silicon (n-a-Si) layer has been optimized in terms of doping level and thickness. The optimized intrinsic amorphous silicon (i-a-Si) layer should be as thin as possible with an energy band gap (E_g) larger than 1.4 eV. In addition, the simulations concerning interface defects strongly suggest that FSF is essential when the front surface is not passivated perfectly. Without FSF, the IBC-HJ solar cells may become more sensitive to interface defect density.     

Fabrication of microcrystalline silicon solar cells on a SnO2 coated substrate using seed layer insertion

We fabricated hydrogenated microcrystalline silicon (μc-Si:H) solar cells on SnO₂ coated glass using a seed layer insertion technique. Since rich hydrogen atoms from the μc-Si:H deposition process degrade the SnO₂ layer, we applied p-type hydrogenated amorphous silicon (p-a-Si:H) as a window layer. To grow the μc-Si:H layer on the p-a-Si:H window layer, we developed a seed layer insertion method. We inserted the seed layer between the p-a-Si:H layer and intrinsic bulk μc-Si:H. This seed layer consists of a thin hydrogen diluted silicon buffer layer and a naturally hydrogen profiled layer. We compared the characteristics of solar cells with and without the seed layer. When the seed layer was not applied, the fabricated cell showed the characteristics of a-Si:H solar cell whose spectral response was in a range of 400-800 nm. Using the seed layer, we achieved a μc-Si:H solar cell with performance of V_oc=0.535 V, J_sc=16.0 mA/cm², FF=0.667, and conversion efficiency=5.7% without any back reflector. The spectral response was in the range of 400-1100 nm. Also, the fabricated device has little substrate dependence, because a-Si:H has weaker substrate selectivity than μc-Si:H.     

Fabrication of Cu2ZnSnS4 films by sulfurization of Cu/ZnSn/Cu precursor layers in sulfur atmosphere for solar cells

Cu₂ZnSnS₄ (CZTS) absorbers were grown by sulfurization of Cu/ZnSn/Cu precursors in sulfur atmosphere. The reaction mechanism of CZTS formation from the precursor was analyzed using XRD and Raman spectroscopy. The films with a single phase CZTS were formed at 560 and 580 °C by sulfurization for 30 min. The film grown at 560 °C showed bi-layer morphology with grooved large grains on the top and dense small grains near the bottom of the film. On the other hand, the film grown at 580 °C showed large grains with grooves that are extended from surface top to bottom of the film. The solar cell fabricated with the CZTS film grown at 560 °C showed the best conversion efficiency of 4.59% for 0.44 cm² with V_oc=0.545 V, J_sc=15.44 mA/cm², and FF=54.6. We found that further improvement of the microstructure of CZTS films can increase the efficiency of CZTS solar cells.     

ZnO interface layer and CO2 plasma treatment for improving efficiency of micromorph silicon solar cells

We have developed zinc oxide (ZnO) film and CO₂ plasma treatment for the use as an intermediate layer between top and bottom cell in order to improve performance of micromorph silicon solar cells. The CO₂ plasma treatment was performed by very high frequency plasma-enhanced chemical vapor deposition (VHF PECVD) technique, and the ZnO interface layer was deposited by DC-magnetron sputtering method. Effects of both techniques on the cell performance were comparatively investigated. We found that the ZnO interface layer and CO₂ plasma treatment were effective in enhancing V_oc, J_sc as well as FF of the cells as the same. The micromorph solar cells using an optimized ZnO interface layer and the CO₂ plasma treatment indicated initial conversion efficiency of 11.4% and 11.2%, respectively. Experimental results indicated that the CO₂ plasma treatment technique is more suitable for using in cell fabrication process than the ZnO interface layer since it is simpler and has no negative impact of possible shunts.     

Theoretical investigation on the absorption enhancement of the crystalline silicon solar cells by pyramid texture coated with SiNx:H layer

The absorption enhancement of the crystalline silicon (c-Si) solar cells by pyramid texture coated with SiN_x:H layer was investigated by theoretical simulation via rigorous coupled-wave analysis (RCWA). It was found that in order to maximize the spectrally weighted absorptance of the solar cells for the Air Mass 1.5 (AM1.5) solar spectrum (A_AM1.5), the required pyramid size (d) was dependent on the thickness of the c-Si substrate. The thinner the c-Si substrate is, the larger the pyramids should be. Pyramids with d > 0.5 μm can make A_AM1.5 maximal if the c-Si substrate thickness is larger than 50 μm. But d > 1.0 μm is needed when the c-Si substrate thickness is less than 25 μm. If the c-Si substrate is thinner than 5 μm, even d > 4.0 μm is required. The underlying mechanism was analyzed according to the diffraction theory. The pyramid texture acts as not only an antireflective (AR) component, but also a light trapping element. Then, the optimized refractive index and the thickness of SiN_x:H layer to further enhance the absorption were given out. The potential solar cell efficiency was also estimated.     

A study of stabilization of P3HT/PCBM organic solar cells by photochemical active TiOx layer

We describe a study of the stabilization behavior of P3HT/PCBM organic solar cells under air and UV irradiation using a 20 nm thin TiO_x protection layer made by partial hydrolysis of a Ti-alkoxide and spin coating in air. Data on the degradation of solar cell performance under air and under UV exposure are presented indicating that significant improvements are observed with TiO_x layer protection. The protection mechanism has been investigated by transmission IR and UV spectroscopy and by ESR spectroscopy. The results of this study suggest how sol-gel derived TiO_x films containing organic functionalities serve as effective passivation films for protection from oxygen when excited by photons, where the photooxidation of the bound organic moieties causes oxygen gas scavenging.     

Efficiency enhancement of polymer solar cells by incorporating a self-assembled layer of silver nanodisks

We report the efficiency enhancement of polymer solar cells by incorporating a silver nanodisks' self-assembled layer, which was grown on the indium tin oxide (ITO) surface by the electrostatic interaction between the silver particles and modified ITO. Polymer solar cells with a structure of ITO (with silver nanodisks)/poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) (Clevious P VP AI 4083)/poly(3-hexylthiophene):[6,6]-phenyl-C61 butyric acid methyl ester (P3HT:PC₆₁BM)/LiF/Al exhibited an open circuit voltage (V_OC) of 0.61±0.01 V, short-circuit current density (J_SC) of 9.24±0.09 mA/cm², a fill factor (FF) of 0.60±0.01, and power conversion efficiency (PCE) of 3.46±0.07% under one sun of simulated air mass 1.5 global (AM1.5G) irradiation (100 mW/cm²). The PCE was increased from 2.72±0.08% of the devices without silver nanodisks to 3.46±0.07%, mainly from the improved photocurrent density as a result of the excited localized surface plasmon resonance (LSPR) induced by the silver nanodisks.     

Identification of PV solar cells and modules parameters using the genetic algorithms: Application to maximum power extraction

In this paper, we propose to perform a numerical technique based on genetic algorithms (GAs) to identify the electrical parameters (I_s, I_ph, R_s, R_sh, and n) of photovoltaic (PV) solar cells and modules. These parameters were used to determine the corresponding maximum power point (MPP) from the illuminated current-voltage (I-V) characteristic. The one diode type approach is used to model the AM1.5 I-V characteristic of the solar cell. To extract electrical parameters, the approach is formulated as a non convex optimization problem. The GAs approach was used as a numerical technique in order to overcome problems involved in the local minima in the case of non convex optimization criteria. Compared to other methods, we find that the GAs is a very efficient technique to estimate the electrical parameters of PV solar cells and modules. Indeed, the race of the algorithm stopped after five generations in the case of PV solar cells and seven generations in the case of PV modules. The identified parameters are then used to extract the maximum power working points for both cell and module.     

Temperature dependence of Cu2ZnSn(SexS1−x)4 monograin solar cells

The temperature dependence of open-circuit voltage (V_oc), short-circuit current (I_sc), fill factor (FF), and relative efficiency of monograin Cu₂ZnSn(Se_xS_1−x)₄ solar cell was measured. The light intensity was varied from 2.2 to 100 mW/cm² and temperatures were in the range of T = 175-300 K. With a light intensity of 100 mW/cm²dV_oc/dT was determined to be −1.91 mV/K and the dominating recombination process at temperatures close to room temperature was found to be related to the recombination in the space-charge region. The solar cell relative efficiency decreases with temperature by 0.013%/K. Our results show that the diode ideality factor n does not show remarkable temperature dependence and slightly increases from n = 1.85 to n = 2.05 in the temperature range between 175 and 300 K.     

The effect of a blocking layer on the photovoltaic performance in CdS quantum-dot-sensitized solar cells

In order to reduce the surface recombination at the interface between the fluorine-doped tin oxide (FTO) substrate and the polysulfide electrolyte in CdS quantum-dot-sensitized solar cells (QDSCs), compact TiO₂ is deposited on the FTO electrode by sputtering. The TiO₂-coated CdS-sensitized solar cell exhibits enhanced power-conversion efficiency (0.52%) compared with a bare CdS-sensitized solar cell (0.23%). Charge-transfer kinetics are analyzed by impedance spectroscopy, open-circuit decay, and cyclic voltammetry. The TiO₂ layer deposited on the FTO substrate acts as a blocking layer, which plays a significant role in reducing the electron back transfer from the FTO to the polysulfide electrolyte. Interestingly, with respect to the incident photon-to-current conversion efficiency (IPCE) data, asymmetric enhancement is observed from the sample with a thicker blocking layer. This is because CdS quantum dots absorb ultraviolet light completely with the TiO₂ layer because of the high extinction coefficient of the CdS quantum dots compared with dye molecules.     

Electrical and optical properties of point-contacted a-Si:H/c-Si heterojunction solar cells with patterned SiO2 at the interface

We report on the electrical and optical characteristics of a-Si:H/c-Si heterojunction solar cells with point-contact junction via patterned SiO₂ layer at the interface. The new structure showed improved electrical properties, having a smaller leakage current and a larger shunt resistance. The electrical conduction of the point-contacted samples followed the diffusion dominant process with bulk recombination, but the control samples without SiO₂ showed the space-charge region recombination dominant process. The point-contacted samples showed increased internal quantum efficiency in the bulk region, but decreased internal quantum efficiency in the surface region. As the distance between the holes decreased, the point-contacted solar cells showed an improved efficiency with a larger fill-factor but smaller open-circuit voltage and short-circuit current.     

Escherichia coli hydrogenase 4 (hyf) and hydrogenase 2 (hyb) contribution in H2 production during mixed carbon (glucose and glycerol) fermentation at pH 7.5 and pH 5.5

Molecular hydrogen (H₂) production by Escherichia coli was studied during mixed carbon sources (glucose and glycerol) fermentation at pH 7.5 and pH 5.5. H₂ production rate (V_H2) by bacterial cells grown on mixed carbon was assayed with either adding glucose (glucose assay) or glycerol (glycerol assay) and compared with the cells grown on sole carbon (glucose or glycerol only) and appropriately assayed. Wild type cells grown on mixed carbon, in the assays with adding glucose, produced H₂ at pH 7.5 with the same level as in the cells grown on glucose only. At pH 7.5 V_H2 in fhlA single and fhlA hyfG double mutants decreased ∼6.5 and ∼7.9 fold, respectively. In wild type cells grown on mixed carbon V_H2 at pH 5.5 was lowered ∼2 fold, compared to the cells grown on glucose only. But in hyfG and hybC single mutants V_H2 was decreased ∼2 and ∼1.6 fold, respectively. However, at pH 7.5, in the assays with glycerol, V_H2 was low, when compared to the cells grown on glycerol only. At pH 5.5 in the assays with glycerol V_H2 was absent. Moreover, V_H2 in wild type cells was inhibited by 0.3 mM N,N^′-dicyclohexylcarbodiimide (DCCD), an inhibitor of the F₀F₁-ATPase, in a pH dependent manner. At pH 7.5 in wild type cells V_H2 was decreased ∼3 fold but at pH 5.5 the inhibition was ∼1.7 fold. At both pHs in fhlA mutant V_H2 was totally inhibited by DCCD. Taken together, the results obtained indicate that at pH 7.5, in the presence of glucose, glycerol can also be fermented. They point out that Hyd-4 mainly and Hyd-2 to some extent contribute in H₂ production by E. coli during mixed carbon fermentation at pH 5.5 whereas Hyd-1 is only responsible for H₂ oxidation.     

Oxygen-dependent enhancement of hydrogen production by engineering bacterial hemoglobin in Escherichia coli

H₂ production under aerobic conditions has been proposed as an alternative method to overcome the fundamentally low yield of H₂ production by fermentative bacteria by maximizing the number of electrons that are available for H₂. Here, we engineered Vitreoscilla hemoglobin (VHb) in Escherichia coli to study the effects of this versatile oxygen (O₂)-binding protein on oxic H₂ production in a closed batch system that was supplemented with glucose. The H₂ yields that were obtained with the VHb-expressing E. coli were greatly enhanced in comparison to the negative control cells in culture that started with high O₂ tensions. The formate hydrogen lyase (FHL) activity of oxically cultured, VHb-expressing cells was also much higher than that of the negative control cells. Through inhibitor studies and time-course experiments, VHb was shown to contribute to the improved H₂ yield primarily by increasing the efficiency of cellular metabolism during the aerobic phase before the onset of H₂ production and not by working as an O₂-scavenger during H₂ production. This new approach allowed more substrate to remain to be further utilized for the production of more H₂ from limited resources. We expect that VHb can be successfully engineered in potential aerobic H₂-producing microbial systems to enhance the overall H₂ production yield. In addition, the remarkably high FHL activity of oxically grown, VHb-expressing cells may make this engineered strain an attractive whole-cell biocatalyst for converting formate to H₂.     

Experimental investigation of Cu(In1−x,Gax)Se2/Zn(O1−z,Sz) solar cell performance

In this study we investigate the performance of Cu(In_1−x,Ga_x)Se₂/Zn(O_1−z,S_z) solar cells by changing the gallium content of the absorber layer in steps from CuInSe₂ to CuGaSe₂ and at each step vary the sulfur content of the Zn(O,S) buffer layer. By incorporating more or less sulfur into the Zn(O,S) buffer layer it is possible to change its morphology and band gap energy. Surprisingly, the best solar cells with Zn(O,S) buffer layers in this study are found for close to or the same Zn(O,S) buffer layer composition for all absorber Ga compositions. In comparison to their CdS references the best solar cells with Zn(O,S) buffer layers have slightly lower open circuit voltage, V_oc, lower fill factor, FF, and higher short circuit current density, J_sc, which result in comparable or slightly lower conversion efficiencies. The exception to this trend is the CuGaSe₂ solar cells, where the best devices with Zn(O,S) have substantially lowered efficiency compared with the CdS reference, because of lower V_oc, FF and J_sc. X-ray photon spectroscopy and X-ray diffraction measurements show that the best Zn(O,S) buffer layers have similar properties independent of the Ga content. In addition, energy dispersive spectroscopy scans in a transmission electron microscope show evidence of lateral variations in the Zn(O,S) buffer layer composition at the absorber/buffer layer interface. Finally, a hypothesis based on the results of the buffer layer analysis is suggested in order to explain the solar cell parameters.